This invention was made with government support under contract number DE-AC06-76RLO 1830, awarded by the U.S. Department of Energy. The government has certain rights in the invention.
A substantial number of ground contaminated areas exist, especially as the result of industrial disposal, which either threaten populated areas or which cannot be used for conventional purposes Soil heating techniques have been proposed for treating contaminated soils containing volatile or semi-volatile organics, such as dioxins, PCB's, and light hydrocarbons. Known methods of heating the soil are relatively cumbersome and expensive, or are incapable of heating the soil itself to the desired depth for removing large quantities of contaminants. For example, radio frequency heating can be expensive and furthermore is capable of treating only a superficial region of the soil.
In Brouns et al U.S. Pat. No. 4,376,598, in situ vitrification of soil is described wherein sufficient electrical energy is applied via electrodes in the ground for converting the soil itself to a conductive, i.e., liquid, state which is then allowed to harden into a vitrified mass. Although, as a result of the intense heat generated in the vitrification process, volatile materials can be driven off or pyrolyzed, the electrical power requirements in heating the soil are reduced in accordance with the present invention for the purpose of volatilizing or pyrolyzing organic materials.
In accordance with the present invention, soils are heated electrically to temperatures substantially lower than those employed for vitrifying the soil. A ground region is heated to a temperature between 100.degree. C. and 1200.degree. C. to volatilize and/or pyrolyze undesired material. For the most part a steady source of resistive heating power, as employed for in situ vitrification, is neither required nor desirable for controlled temperature heating of soil. Although such AC or DC resistive heating can be employed to reduce soil moisture, this type of heating is not acceptable when the soil begins to dry. For dry soil, or when the moist soil tends to dry out in process, the soil becomes a poor electrical conductor such that resistive heating becomes ineffective.
If sufficient DC or AC voltage is applied between electrodes to produce continuous arcing, large or artificially cooled electrodes may be required, and once a gas plasma forms, higher currents are drawn from the power supply to maintain a reasonable power level. That is, as long as current is flowing, the voltage between electrodes then remains substantially lower than employed to initiate the current in the first place because gases in the electrical path become ionized to a more conductive state.